Field of the Invention
The present invention is in the technical field of fiber and microstructure fabrication. In some embodiments, the invention also relates to the permanent or semi-permanent recording and/or reading of information on or within fibers and microstructures. In some embodiments, the invention also relates to the production of certain functionally-shaped and engineered short fiber and microstructure materials. In some embodiments, the invention may also utilize laser beam profiling to enhance fiber and microstructure fabrication.
In some aspects, this invention generally relates to production of fibers that are commonly used to reinforce composite materials. Frequently, short strands of fiber are cut from longer rolls of fiber, wire, or rolled strips to predetermined lengths, and these are then added to composite matrix materials in random or ordered arrangements. These fibers are known as “chopped fiber” in the industry and are used in a broad range of applications, from carbon-fiber reinforced polymers to sprayed-on metallic-fiber-reinforced insulation, to polymer-fiber reinforced concretes. In the composites industry, long strands of fiber are also spooled/joined into tow or ropes, which can then be used to create “fiber layups” and reinforce composite materials.
Very often, fibers provide increased strength to a composite material, while a surrounding matrix material possesses complementary properties. The overall strength of a composite material depends on both the fiber and matrix properties, but usually strength is compromised when the fibers can slip excessively relative to the matrix. Thus, one of the greatest challenges associated with fiber-reinforcement of composites is optimizing the “pull-out” strength of fibers within a matrix material. Traditionally, this has been done by: (1) increasing the adhesion or bonding strength at the interface between the fibers and matrix material, or (2) increasing the surface area for contact between the two materials.
However, the concept of optimal fibers whose shapes are engineered to minimize pull-out while allowing the composite to remain flexible and tough is essentially absent from existing techniques. One reason for this is simply that current manufacturing methods presume that the raw fiber or wire-based materials are derived from drawing the fiber material through dies; in the case of metallic strip, they are often cut from rolled metal sheeting. For many materials, it is difficult to modify the cross-section of fibers dynamically using dies or rolling processes. Thus carbon-fiber manufacturers largely produce circular cross-section carbon fiber, and metallic fiber is often chopped from cylindrical wire—all with constant cross-sections. Of course, ductile/metallic wire/strip can be rolled, indented, or bent mechanically to change its shape, but this is not practical for many higher strength (often brittle) materials that are desired for high-performance composites, such as carbon, silicon carbide, silicon nitride, boron, or boron nitride, etc. Forming processes increase the overall expense of the process and are usually limited in the potential geometries that can be created. Thus, a method of modulating the cross-section versus length of reinforcing fibers is very desirable, especially when optimal reinforcing geometries can be created.
Note that pull-out strength is not the only parameter that must be optimized for reinforcing fiber. In many situations, it is also useful to have fibers that are designed to bend, flex, twist, etc. without failure or delamination from the matrix. Creating fibers in shapes that give more isotropic properties are desired in many applications. For example, carbon fibers may have high tensile strengths in one direction while possessing poor compressive or shear strengths. This derives largely from the way in which they are processed from continuous strands—which provides particular anisotropic microstructures/orientations along the axis of the fiber. However, changing the nominal geometric orientation of the fiber itself—into non-linear geometries—can greatly improve the shear and flexure properties of the resulting composite material. This is difficult to accomplish through traditional fiber manufacturing methods.
Hyperbaric laser chemical vapor deposition has been traditionally used with simple Gaussian Laser Beam profiles to grow free-standing, three-dimensional fibers from a wide variety of materials. A Gaussian beam profile is brightest in the center of the beam, and the intensity tails off radially with distance from the central axis of the beam according to:I(r)=Io*Exp(−2r2/wo2)When focused by a positive lens onto a surface, such a Gaussian beam generates a focal spot that also possesses this same Gaussian distribution. Thus, when fibers are grown by HP-LCVD using a Gaussian beam, the fiber is heated most at the center of the fiber, but the absorbed energy drops radially. Provided the thermal conductivity of the fiber material is high, the fiber dimensions are small, and the growth rate slow, this absorbed thermal energy can conduct rapidly across the fiber tip, allowing the temperature profile within the reaction zone at the fiber tip to be fairly uniform. However for moderate-to-low thermal conductivity materials, the center of the fiber is usually at a much higher temperatures than the fiber edges.
This creates several problems for rapid fiber growth: First, as the phase and composition of the material that is grown can depend strongly on temperature, a non-uniform temperature distribution can create two or more phases or compositions of materials in the fiber. For example during the deposition of carbon fibers from ethylene, at least four possible material phases can be deposited: amorphous/fine-grained carbon, graphitic carbon, nodular carbon, and diamond-like carbon, depending on the reaction temperature. Thus, with a Gaussian laser beam at moderate laser powers, it is very common to grow carbon fibers that possess a graphitic carbon core, with an amorphous or fine-grained carbon coating. This is illustrated in FIG. 37(c). The graphitic core often consists of parabolic- or Gaussian-shaped graphite shells that are centered on the fiber axis, and run outwards toward the fiber exterior. This material configuration provides strength radially, but is not very strong along the fiber axis. This leaves the fibers with very little tensile strength along their primary axis—which is crucial for fiber reinforcement applications. To be most useful/competitive commercially, the carbon fibers grown by HP-LCVD should either be entirely amorphous/glassy, or have graphitic planes running in the same direction as the fiber axis to add strength along that direction.
In addition, many desired fibers are binary or ternary compounds or alloys that are deposited using two or more precursors. Each precursor generally exhibits its own deposition kinetics and activation energy, and hence deposits differently vs. temperature than the other precursors. When a single-temperature is present, this is not generally an issue, as the concentration of the gas-phase precursors can compensate for the difference in deposition kinetics. However, in a temperature gradient, this will lead to a varying composition of the deposit elements within the fiber. For a Gaussian beam, this means that a radial compositional gradient will exist for two or more precursors. Sometimes this can be of advantage (e.g. obtaining a protective coating over a core material in a single-step). However, often a single composition within the fiber is desired.